- First, plug in an 8GB (or bigger) USB drive, and use Disk Utility to erase it
 - If you use the default settings, you should wind up with a blank drive at 
/Volumes/Untitled. 
With that volume in place, and with the macOS installer sitting in /Applications/Install\ macOS\ [VERSION].app,
run the following command in your terminal to create a bootable install media (for Sierra):
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction